arXiv subjects

C. M. Casey

Publications and source records attributed to C. M. Casey.

At least 37 records · Page 2Linked to original sources

arXiv2021

COSMOS2020: A panchromatic view of the Universe to $z\sim10$ from two complementary catalogs

The Cosmic Evolution Survey (COSMOS) has become a cornerstone of extragalactic astronomy. Since the last public catalog in 2015, a wealth of new imaging and spectroscopic data has been collected in the COSMOS field. This paper describes the collection, processing, and analysis of this new imaging data to produce a new reference photometric redshift catalog. Source detection and multi-wavelength photometry is performed for 1.7 million sources across the $2\,\mathrm{deg}^{2}$ of the COSMOS field, $\sim$966,000 of which are measured with all available broad-band data using both traditional aperture photometric methods and a new profile-fitting photometric extraction tool, The Farmer, which we have developed. A detailed comparison of the two resulting photometric catalogs is presented. Photometric redshifts are computed for all sources in each catalog utilizing two independent photometric redshift codes. Finally, a comparison is made between the performance of the photometric methodologies and of the redshift codes to demonstrate an exceptional degree of self-consistency in the resulting photometric redshifts. The $i<21$ sources have sub-percent photometric redshift accuracy and even the faintest sources at $25<i<27$ reach a precision of $5\,\%$. Finally, these results are discussed in the context of previous, current, and future surveys in the COSMOS field. Compared to COSMOS2015, reaches the same photometric redshift precision at almost one magnitude deeper. Both photometric catalogs and their photometric redshift solutions and physical parameters will be made available through the usual astronomical archive systems (ESO Phase 3, IPAC IRSA, and CDS).

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arXiv2021

Measurements of the dust properties in z~1-3 SMGs with ALMA

We present Atacama Large Millimetre Array (ALMA) 2mm continuum observations of a complete and unbiased sample of 99 870micron-selected sub-millimeter galaxies (SMGs) in the Extended Chandra Deep Field South (ALESS). Our observations of each SMG reach average sensitivities of 53 microJy/beam. We measure the flux densities for 70 sources, for which we obtain a typical 870micron-to-2mm flux ratio of 14 +/- 5. We do not find a redshift dependence of this flux ratio, which would be expected if the dust emission properties of our SMGs were the same at all redshifts. By combining our ALMA measurements with existing Herschel/SPIRE observations, we construct a (biased) subset of 27 galaxies for which the cool dust emission is sufficiently well sampled to obtain precise constraints on their dust properties using simple isothermal models. Thanks to our new 2mm observations, the dust emissivity index is well-constrained and robust against different dust opacity assumptions. The median dust emissivity index of our SMGs is $β\simeq1.9\pm0.4$, consistent with the emissivity index of dust in the Milky Way and other local and high-redshift galaxies, as well as classical dust grain model predictions. We also find a negative correlation between the dust temperature and $β$, similar to low-redshift observational and theoretical studies. Our results indicate that $β\simeq2$ in high-redshift dusty star-forming galaxies, implying little evolution in dust grain properties between our SMGs and local dusty galaxy samples, and suggesting these high-mass and high-metallicity galaxies have dust reservoirs driven by grain growth in their ISM.

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arXiv2021

Early Science with the Large Millimeter Telescope: a 1.1 mm AzTEC Survey of Red-$Herschel$ dusty star-forming galaxies

We present LMT/AzTEC 1.1mm observations of $\sim100$ luminous high-redshift dusty star-forming galaxy candidates from the $\sim600\,$sq.deg $Herschel$-ATLAS survey, selected on the basis of their SPIRE red far-infrared colours and with $S_{500μ\rm m}=35-80$ mJy. With an effective $θ_{\rm FWHM}\approx9.5\,$ arcsec angular resolution, our observations reveal that at least 9 per cent of the targets break into multiple systems with SNR $\geq 4$ members. The fraction of multiple systems increases to $\sim23\,$ per cent (or more) if some non-detected targets are considered multiples, as suggested by the data. Combining the new AzTEC and deblended $Herschel$ photometry we derive photometric redshifts, IR luminosities, and star formation rates. While the median redshifts of the multiple and single systems are similar $(z_{\rm med}\approx3.6)$, the redshift distribution of the latter is skewed towards higher redshifts. Of the AzTEC sources $\sim85\,$ per cent lie at $z_{\rm phot}>3$ while $\sim33\,$ per cent are at $z_{\rm phot}>4$. This corresponds to a lower limit on the space density of ultra-red sources at $4 4$ galaxies.

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arXiv2021

COALAS: I. ATCA CO(1-0) survey and luminosity function in the Spiderweb protocluster at z=2.16

We report a detailed CO(1-0) survey of a galaxy protocluster field at $z=2.16$, based on 475 hours of observations with the Australia Telescope Compact Array. We constructed a large mosaic of 13 individual pointings, covering an area of 21 arcmin$^2$ and $\pm6500$ km/s range in velocity. We obtain a robust sample of 46 CO(1-0) detections spanning $z=2.09-2.22$, constituting the largest sample of molecular gas measurements in protoclusters to date. The CO emitters show an overdensity at $z=2.12-2.21$, suggesting a galaxy super-protocluster or a protocluster connected to large-scale filaments with ~120 cMpc size. We find that 90% CO emitters have distances $>0'.5-4'$ to the center galaxy, indicating that small area surveys would miss the majority of gas reservoirs in similar structures. Half of the CO emitters have velocities larger than escape velocities, which appears gravitationally unbound to the cluster core. These unbound sources are barely found within the $R_{200}$ radius around the center, which is consistent with a picture in which the cluster core is collapsed while outer regions are still in formation. Compared to other protoclusters, this structure contains relatively more CO emitters with relatively narrow line width and high luminosity, indicating galaxy mergers. We use these CO emitters to place the first constraint on the CO luminosity function and molecular gas density in an overdense environment. The amplitude of the CO luminosity function is 1.6$\pm$0.5 orders of magnitudes higher than observed for field galaxy samples at $z\sim2$, and one order of magnitude higher than predictions for galaxy protoclusters from semi-analytical SHARK models. We derive a high molecular gas density of $0.6-1.3\times10^{9}$ $M_\odot$ cMpc$^{-3}$ for this structure, consistent with predictions for cold gas density of massive structures from hydro-dynamical DIANOGA simulations.

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arXiv2021

The Evolution of the IR Luminosity Function and Dust-obscured Star Formation in the Last 13 Billion Years

We present the first results from the 2mm Mapping Obscuration to Reionization (MORA) survey, the largest ALMA contiguous blank-field survey to-date with a total area of 184 sq. arcmin and the only at 2mm to search for dusty star-forming galaxies (DSFGs). We use the 13 sources detected above 5sigma to estimate the first ALMA galaxy number counts at this wavelength. These number counts are then combined with the state-of-the-art galaxy number counts at 1.2mm and 3mm and with a backward evolution model to place constraints on the evolution of the IR luminosity function and dust-obscured star formation in the last 13 billion years. Our results suggest a steep redshift evolution on the space density of DSFGs and confirm the flattening of the IR luminosity function at faint luminosities, with a slope of $α_{LF} = -0.42^{+0.02}_{-0.04}$. We conclude that the dust-obscured component, which peaks at z=2-2.5, has dominated the cosmic history of star formation for the past ~12 billion years, back to z~4. At z=5, the dust-obscured star formation is estimated to be ~35% of the total star formation rate density and decreases to 25%-20% at z=6-7, implying a minor contribution of dust-enshrouded star formation in the first billion years of the Universe. With the dust-obscured star formation history constrained up to the end of the epoch of reionization, our results provide a benchmark to test galaxy formation models, to study the galaxy mass assembly history, and to understand the dust and metal enrichment of the Universe at early times.

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arXiv2020

The redshift and star formation mode of AzTEC2: a pair of massive galaxies at $z=4.63$

We combine observations from the Atacama Large Millimeter/submillimeter Array (ALMA) and the NOrthern Extended Millimeter Array (NOEMA) to assess the redshift and to study the star formation conditions in AzTEC2: one of the brightest sub-millimeter galaxies (SMGs) in the COSMOS field ($S_{\rm 1.1mm}=10.5\pm1.4$mJy). Our high-resolution observations confirm that AzTEC2 splits into two components (namely AzTEC2-A and AzTEC2-B) for which we detect [C$\,$II] and $^{12}$CO(5$\to$4) line emission, implying a redshift of $4.626\pm0.001$ ($4.633\pm0.001$) for AzTEC2-A (AzTEC2-B) and ruling out previous associations with a galaxy at $z\sim1$. We use the $^{12}$CO(5$\to$4) line emission and adopt typical SMG-like gas excitation conditions to estimate the molecular gas mass, which is $M_{\rm gas}(α_{\rm CO}/2.5)=2.1\pm0.4 \times10^{11}{\rm M}_\odot$ for AzTEC2-A, and a factor four lower for AzTEC2-B. With the infrared-derived star formation rate of AzTEC2-A ($1920\pm100 \,M_\odot{\rm \, yr}^{-1}$) and AzTEC2-B ($710\pm 35\,M_\odot{\rm \,yr}^{-1}$), they both will consume their current gas reservoir within $(30-200)\,$Myr. We find evidence of a rotation-dominated [C$\,$II] disk in AzTEC2-A, with a de-projected rotational velocity of $v_{\rm rot}(i=39^\circ)=660\pm130{\rm \,km\,s}^{-1}$, velocity dispersion $\lesssim100{\rm \,km\,s}^{-1}$, and dynamical mass of $M_{\rm dyn}(i=39^\circ)=2.6^{+1.2}_{-0.9}\times10^{11}\,M_\odot$. We propose that an elevated gas accretion rate from the cosmic web might be the main driver of the intense levels of star formation in AzTEC2-A, which might be further enhanced by gravitational torques induced by its minor companion (AzTEC2-B). These results strengthen the picture whereby the population of single-dish selected SMGs is rather heterogeneous, including a population of pairs of massive, highly-active galaxies in a pre-coalescence phase.

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arXiv2019

First detection of the [OI] 63-um emission from a redshift 6 dusty galaxy

We report a ground-based detection of the [OI] 63-um line in a z=6.027 gravitationally lensed dusty star-forming galaxy (DSFG) G09.83808 using the APEX SEPIA 660 receiver, the first unambiguous detection of the [OI]63 line beyond redshift 3, and the first obtained from the ground. The [OI]63 line is robustly detected at 22$\pm$5 Jy km s$^{-1}$, corresponding to an intrinsic (de-lensed) luminosity of $(5.4\pm1.3)\times10^{9}$ L$_\odot$. With the [OI]63/[CII] luminosity ratio of 4, the [OI]63 line is the main coolant of the neutral gas in this galaxy, in agreement with model predictions. The high [OI]63 luminosity compensates for the pronounced [CII] deficit ([CII]/FIR$\simeq4\times10^{-4}$). Using photon-dominated region models, we derive a source-averaged gas density $n=10^{4.0}$ cm$^{-3}$, and far-UV field strength $G=10^4 G_0$, comparable to the z=2-4 DSFG population. If G09.83808 represents a typical high-redshift DSFG, the [OI]63 line from z=6 non-lensed DSFGs should be routinely detectable in ALMA Band 9 observations with $\sim$15 min on-source, opening a new window to study the properties of the earliest DSFGs.

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arXiv2019

On the Gas Content, Star Formation Efficiency, and Environmental Quenching of Massive Galaxies in Proto-Clusters at z~2.0-2.5

We present ALMA Band 6 (nu=233GHz, lambda=1.3mm) continuum observations towards 68 'normal' star-forming galaxies within two Coma-like progenitor structures at z=2.10 and 2.47, from which ISM masses are derived, providing the largest census of molecular gas mass in overdense environments at these redshifts. Our sample comprises galaxies with a stellar mass range of 1x10^9M_sun - 4x10^11M_sun with a mean M_*~6x10^10M_sun. Combining these measurements with multiwavelength observations and SED modeling, we characterize the gas mass fraction and the star formation efficiency, and infer the impact of the environment on galaxies' evolution. Most of our detected galaxies (~70%) have star formation efficiencies and gas fractions similar to those found for coeval field galaxies and in agreement with the field scaling relations. However, we do find that the proto-clusters contain an increased fraction of massive, gas-poor galaxies, with low gas fractions (f_gas<6-10%) and red rest-frame ultraviolet/optical colors typical of post-starburst and passive galaxies. The relatively high abundance of passive galaxies suggests an accelerated evolution of massive galaxies in proto-cluster environments. The large fraction of quenched galaxies in these overdense structures also implies that environmental quenching takes place during the early phases of cluster assembly, even before virialization. From our data, we derive a quenching efficiency of E_q~0.45 and an upper limit on the quenching timescale of T_q<1Gyr.

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arXiv2019

The IRAM/GISMO two-millimeter survey in the COSMOS field

We present deep continuum observations at a wavelength of 2mm centered on the COSMOS field using the Goddard IRAM Superconducting Millimeter Observer (GISMO) at the IRAM 30m-telescope. These data constitute the widest deep 2mm survey to-date, reaching a uniform $σ\sim0.23$mJy beam$^{-1}$ sensitivity over $\sim250$arcmin$^2$ at $\sim\,24''$ resolution. We detect four sources at high significance (S/N$\,\geq\,$4.4) with an expected number of false detection of 0.09 sources, and five sources at $4.4>\,$S/N$\,\geq\,3.7$ with an expected number of false detection of 1.65 sources. Combined with deep GISMO observations in GOODS-N, we constrain the 2mm number counts over one decade in flux density. These measurements agree with most galaxy evolution models tested here, except those with large population of dusty star-forming galaxies at $z>7$. Five GISMO sources have counterparts in (sub-)millimeter catalogs available in COSMOS. Their redshifts suggest that all but one lie above $z\sim3$. These four high-redshift ($z>3$) galaxies have $\tilde{z}=3.9$, SFRs $\sim\,$400 - 1200M$_{\odot}\,$yr$^{-1}$ and $M_{\rm dust}\sim10^{9.5}\,$M$_{\odot}$. They provide a relatively complete selection ($\sim66\%$) of the most luminous ($L_{\rm IR}>10^{12.6}\,$L$_{\odot}$) and highest redshift ($z>3$) galaxies detected within our survey area by AzTEC at 1.1mm. We thus conclude that 2mm surveys favor the selection of massive, vigorously star-forming, high-redshift galaxies. This is corroborated by GISMO-C4, a source with a low false detection probability ($\sim\,6.2\%$), for which the absence of a (sub-)millimeter counterpart supports a high redshift origin ($z\gtrsim3$).

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arXiv2018

Constraining the volume density of Dusty Star-Forming Galaxies through the first 3mm Number Counts from ALMA

We carry out a blind search of 3mm continuum sources using the ALMA Science Archive to derive the first galaxy number counts at this wavelength. The analyzed data are drawn from observations towards three extragalactic legacy fields: COSMOS, CDF-S, and the UDS comprising more than 130 individual ALMA Band 3 pointings and an effective survey area of ~200 sq. arcmin with a continuum sensitivity that allows for the direct detection of unlensed Dusty Star-Forming Galaxies (DSFGs) dust emission beyond the epoch of reionization. We present a catalog of 16 sources detected at >5sigma with flux densities S_3mm = 60 - 600 uJy from which number counts are derived. These number counts are then used to place constraints on the volume density of DSFGs with an empirical backward evolution model. Our measured 3mm number counts indicate that the contribution of DSFGs to the cosmic star formation rate density at z >~ 4 is non-negligible. This is contrary to the generally adopted assumption of a sharply decreasing contribution of obscured galaxies at z > 4 as inferred by optical and near-infrared surveys. This work demonstrates the power of ALMA 3mm observations which can reach outstanding continuum sensitivities during typical spectral line science programs. Further constraints on 3mm-selected galaxies will be essential to refine models of galaxy formation and evolution as well as models of early Universe dust production mechanisms.

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arXiv2017

AMI-LA Observations of the SuperCLASS Super-cluster

We present a deep survey of the SuperCLASS super-cluster - a region of sky known to contain five Abell clusters at redshift $z\sim0.2$ - performed using the Arcminute Microkelvin Imager (AMI) Large Array (LA) at 15.5$~$GHz. Our survey covers an area of approximately 0.9 square degrees. We achieve a nominal sensitivity of $32.0~μ$Jy beam$^{-1}$ toward the field centre, finding 80 sources above a $5σ$ threshold. We derive the radio colour-colour distribution for sources common to three surveys that cover the field and identify three sources with strongly curved spectra - a high-frequency-peaked source and two GHz-peaked-spectrum sources. The differential source count (i) agrees well with previous deep radio source count, (ii) exhibits no evidence of an emerging population of star-forming galaxies, down to a limit of 0.24$~$mJy, and (iii) disagrees with some models of the 15$~$GHz source population. However, our source count is in agreement with recent work that provides an analytical correction to the source count from the SKADS Simulated Sky, supporting the suggestion that this discrepancy is caused by an abundance of flat-spectrum galaxy cores as-yet not included in source population models.

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arXiv2017

Herschel and Hubble study of a lensed massive dusty starbursting galaxy at $z\sim3$

We present the results of combined deep Keck/NIRC2, HST/WFC3 near-infrared and Herschel far infrared observations of an extremely star forming dusty lensed galaxy identified from the Herschel Astrophysical Terahertz Large Area Survey (H-ATLAS J133542.9+300401). The galaxy is gravitationally lensed by a massive WISE identified galaxy cluster at $z\sim1$. The lensed galaxy is spectroscopically confirmed at $z=2.685$ from detection of $\rm {CO (1 \rightarrow 0)}$ by GBT and from detection of $\rm {CO (3 \rightarrow 2)}$ obtained with CARMA. We use the combined spectroscopic and imaging observations to construct a detailed lens model of the background dusty star-forming galaxy (DSFG) which allows us to study the source plane properties of the target. The best-fit lens model provide magnification of $μ_{\rm star}=2.10\pm0.11$ and $μ_{\rm dust}=2.02\pm0.06$ for the stellar and dust components respectively. Multi-band data yields a magnification corrected star formation rate of $1900(\pm200)\,M_{\odot}{\rm yr^{-1}}$ and stellar mass of $6.8_{-2.7}^{+0.9}\times10^{11}\,M_{\odot}$ consistent with a main sequence of star formation at $z\sim2.6$. The CO observations yield a molecular gas mass of $8.3(\pm1.0)\times10^{10}\,M_{\odot}$, similar to the most massive star-forming galaxies, which together with the high star-formation efficiency are responsible for the intense observed star formation rates. The lensed DSFG has a very short gas depletion time scale of $\sim40$ Myr. The high stellar mass and small gas fractions observed indicate that the lensed DSFG likely has already formed most of its stellar mass and could be a progenitor of the most massive elliptical galaxies found in the local Universe.

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arXiv2017

Evolution of Interstellar Medium, Star Formation, and Accretion at High Redshift

ALMA observations of the long wavelength dust continuum are used to estimate the interstellar medium (ISM) masses in a sample of 708 galaxies at z = 0.3 to 4.5 in the COSMOS field. The galaxy sample has known far-infrared luminosities and, hence, star formation rates (SFRs), and stellar masses (M$_{\rm *}$) from the optical-infrared spectrum fitting. The galaxies sample SFRs from the main sequence (MS) to 50 times above the MS. The derived ISM masses are used to determine the dependence of gas mass on redshift, M$_{\rm *}$, and specific SFR (sSFR) relative to the MS. The ISM masses increase approximately 0.63 power of the rate of increase in SFRs with redshift and the 0.32 power of the sSFR/sSFR$_MS$. The SF efficiencies also increase as the 0.36 power of the SFR redshift evolutionary and the 0.7 power of the elevation above the MS; thus the increased activities at early epochs are driven by both increased ISM masses and SF efficiency. Using the derived ISM mass function we estimate the accretion rates of gas required to maintain continuity of the MS evolution ($>100$ \msun yr$^{-1}$ at z $>$ 2.5). Simple power-law dependences are similarly derived for the gas accretion rates. We argue that the overall evolution of galaxies is driven by the rates of gas accretion. The cosmic evolution of total ISM mass is estimated and linked to the evolution of SF and AGN activity at early epochs.

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arXiv2017

Near-Infrared MOSFIRE Spectra of Dusty Star-Forming Galaxies at 0.2<z<4

We present near-infrared and optical spectroscopic observations of a sample of 450$μ$m and 850$μ$m-selected dusty star-forming galaxies (DSFGs) identified in a 400 arcmin$^2$ area in the COSMOS field. Thirty-one sources of the 102 targets were spectroscopically confirmed at $0.2 0.2$) or non-existent photometric redshifts; these have very distinct spectral energy distributions from the remaining DSFGs, suggesting a decoupling of highly obscured and unobscured components. We present a composite rest-frame 4300--7300Å spectrum for DSFGs, and find evidence of 200$\pm$30 km s$^{-1}$ gas outflows. Nebular line emission for a sub-sample of our detections indicate that hard ionizing radiation fields are ubiquitous in high-z DSFGs, even more so than typical mass or UV-selected high-z galaxies. We also confirm the extreme level of dust obscuration in DSFGs, measuring very high Balmer decrements, and very high ratios of IR to UV and IR to H$α$ luminosities. This work demonstrates the need to broaden the use of wide bandwidth technology in the millimeter to the spectroscopic confirmations of large samples of high-z DSFGs, as the difficulty in confirming such sources at optical/near-infrared wavelengths is exceedingly challenging given their obscuration.

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arXiv2017

The AT-LESS CO(1-0) survey of submillimetre galaxies in the Extended Chandra Deep Field South: First results on cold molecular gas in galaxies at z ~ 2

We present the first results from our on-going Australia Telescope Compact Array survey of CO(1-0) in ALMA-identified submillimetre galaxies in the Extended Chandra Deep Field South. Strong detections of CO(1-0) emission from two submillimetre galaxies, ALESS 122.1 (z = 2.0232) and ALESS 67.1 (z = 2.1230), were obtained. We estimate gas masses of M_gas ~ 1.3 \times 10^{11} M_odot and M_gas ~ 1.0 \times 10^{11} M_\odot for ALESS 122.1 and ALESS 67.1, respectively, adopting alpha_CO = 1.0. Dynamical mass estimates from the kinematics of the CO(1-0) line yields M_dyn (sin i)^2 = 2.1 +- 1.1 \times 10^{11} M_odot and (3.2 +- 0.9) \times 10^{11} M_\odot for ALESS 122.1 and ALESS 67.1, respectively. This is consistent with the total baryonic mass estimates of these two systems. We examine star formation efficiency using the L_FIR versus L'_CO(1-0) relation for samples of local ULIRGs and LIRGs, and more distant star-forming galaxies, with CO(1-0) detections. We find some evidence of a shallower slope for ULIRGs and SMGs compared to less luminous systems, but a larger sample is required for definite conclusions. We determine gas-to-dust ratios of 170 +- 30 and 140 +- 30 for ALESS 122.1 and ALESS 67.1, respectively, showing ALESS 122.1 has an unusually large gas reservoir. By combining the 38.1 GHz continuum detection of ALESS 122.1 with 1.4 and 5.5 GHz data, we estimate that the free-free contribution to radio emission at 38.1 GHz is 34 +- 17 microJy, yielding a star formation rate (1400 +- 700 M_\odot yr^{-1}) consistent with that from the infrared luminosity.

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arXiv2016

Deep observations of the Super-CLASS super-cluster at 325 MHz with the GMRT: the low-frequency source catalogue

We present the results of 325 MHz GMRT observations of a super-cluster field, known to contain five Abell clusters at redshift $z \sim 0.2$. We achieve a nominal sensitivity of $34\,μ$Jy beam$^{-1}$ toward the phase centre. We compile a catalogue of 3257 sources with flux densities in the range $183\,μ\rm{Jy}\,-\,1.5\,\rm{Jy}$ within the entire $\sim 6.5$ square degree field of view. Subsequently, we use available survey data at other frequencies to derive the spectral index distribution for a sub-sample of these sources, recovering two distinct populations -- a dominant population which exhibit spectral index trends typical of steep-spectrum synchrotron emission, and a smaller population of sources with typically flat or rising spectra. We identify a number of sources with ultra-steep spectra or rising spectra for further analysis, finding two candidate high-redshift radio galaxies and three gigahertz-peaked-spectrum radio sources. Finally, we derive the Euclidean-normalised differential source counts using the catalogue compiled in this work, for sources with flux densities in excess of $223 \, μ$Jy. Our differential source counts are consistent with both previous observations at this frequency and models of the low-frequency source population. These represent the deepest source counts yet derived at 325 MHz. Our source counts exhibit the well-known flattening at mJy flux densities, consistent with an emerging population of star-forming galaxies; we also find marginal evidence of a downturn at flux densities below $308 \, μ$Jy, a feature so far only seen at 1.4 GHz.

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arXiv2016

ISM masses and the star formation law at Z = 1 to 6 // ALMA observations of dust continuum in 145 galaxies in the COSMOS survey field

ALMA Cycle 2 observations of the long wavelength dust emission in 145 star-forming galaxies are used to probe the evolution of star-forming ISM. We also develop the physical basis and empirical calibration (with 72 low-z and z ~ 2 galaxies) for using the dust continuum as a quantitative probe of interstellar medium (ISM) masses. The galaxies with highest star formation rates (SFRs) at = 2.2 and 4.4 have gas masses up to 100 times that of the Milky Way and gas mass fractions reaching 50 to 80%, i.e. gas masses 1 - 4 times their stellar masses. We find a single high-z star formation law: SFR = 35 M_ mol^0.89 x (1+z)_{z=2}^0.95 x (sSFR)_{MS}^0.23 \msun yr^-1 -- an approximately linear dependence on the ISM mass and an increased star formation efficiency per unit gas mass at higher redshift. Galaxies above the Main Sequence (MS) have larger gas masses but are converting their ISM into stars on a timescale only slightly shorter than those on the MS -- thus these 'starbursts' are largely the result of having greatly increased gas masses rather than and increased efficiency for converting gas to stars. At z $> 1$, the entire population of star-forming galaxies has $\sim$ 2 - 5 times shorter gas depletion times than low-z galaxies. These shorter depletion times indicate a different mode of star formation in the early universe -- most likely dynamically driven by compressive, high-dispersion gas motions -- a natural consequence of the high gas accretion rates.

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arXiv2016

ISM Masses and Star Formation at z = 1 to 6 ALMA Observations of Dust Continuum in 180 Galaxies in COSMOS

ALMA Cycle 2 observations of the long wavelength dust emission in 180 star-forming (SF) galaxies are used to investigate the evolution of ISM masses at z = 1 to 6.4. The ISM masses exhibit strong increases from z = 0 to $\rm $ = 1.15 and further to $\rm $ = 2.2 and 4.8, particularly amongst galaxies above the SF galaxy main sequence (MS). The galaxies with highest SFRs at $\rm $ = 2.2 and 4.8 have gas masses 100 times that of the Milky Way and gas mass fractions reaching 50 to 80\%, i.e. gas masses 1 - 4$\times$ their stellar masses. For the full sample of galaxies, we find a single, very simple SF law: $\rm SFR \propto M_{\rm ISM}^{0.9}$, i.e. a `linear' dependence on the ISM mass -- on and above the MS. Thus, the galaxies above the MS are converting their larger ISM masses into stars on a timescale similar to those on the MS. At z $> 1$, the entire population of star-forming galaxies has $\sim$5 - 10$\times$ shorter gas depletion times ($\sim0.2$ Gyr) than galaxies at low redshift. These {\bf shorter depletion times are due to a different, dominant mode of SF in the early universe} -- dynamically driven by compressive, high dispersion gas motions and/or galaxy interactions. The dispersive gas motions are a natural consequence of the extraordinarily high gas accretion rates which must occur to maintain the prodigious SF.

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